feat: rename file and move to correct folder on metadata edit

After saving metadata with ffmpeg:
1. Title change → rename the mp3 in place to "{01 - }?{sanitized title}.ext"
   (same convention as the downloader uses)
2. Album-artist or album change → compute new {artistFirstPart}/{album}
   directory, move the file there (create dirs as needed)
3. Old album dir is deleted if empty after the move; old artist dir
   is deleted too if it also becomes empty

When only the album name changes (no artist change) the old in-place
folder-rename behaviour is preserved.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/novasonic-server.js

@@ -684,15 +684,52 @@ app.post('/api/edit-metadata', async (req, res) => {
     fs.renameSync(tmpFile, filePath);
     if (artFile) try { fs.unlinkSync(artFile); } catch {}
 
-    // Rename album folder if album changed and folder name matches old album dir
+    // ── Post-save: rename file and/or move to correct artist/album folder ──
-    if (album) {
+    let currentFile = filePath;
-      const dir        = path.dirname(filePath);
+
-      const newDirName = sanitize(album);
+    // 1. Rename file if title changed — convention: {01 - }?{sanitized title}.ext
-      if (path.basename(dir) !== newDirName) {
+    if (title) {
-        const newDir = path.join(path.dirname(dir), newDirName);
+      const safeTitle  = sanitize(title);
-        if (!fs.existsSync(newDir)) {
+      const prefix     = trackNum ? String(trackNum).padStart(2, '0') + ' - ' : '';
-          try { fs.renameSync(dir, newDir); } catch (e) { console.error('[edit-metadata] folder rename:', e.message); }
+      const newName    = `${prefix}${safeTitle}${fileExt}`;
+      const newPath    = path.join(path.dirname(currentFile), newName);
+      if (newPath !== currentFile && !fs.existsSync(newPath)) {
+        try { fs.renameSync(currentFile, newPath); currentFile = newPath; }
+        catch (e) { console.error('[edit-metadata] file rename:', e.message); }
+      }
+    }
+
+    // 2. Move to new artist/album folder when albumArtist or album changed
+    const oldAlbumDir  = path.dirname(currentFile);
+    const oldArtistDir = path.dirname(oldAlbumDir);
+    const newAlbumName  = album       ? sanitize(album) : path.basename(oldAlbumDir);
+    const newArtistName = albumArtist ? sanitize(albumArtist.split(/[,;]/)[0].trim()) : path.basename(oldArtistDir);
+    const newDir = path.join(CONFIG.musicPath, newArtistName, newAlbumName);
+
+    if (newDir !== oldAlbumDir) {
+      try {
+        fs.mkdirSync(newDir, { recursive: true });
+        const destFile = path.join(newDir, path.basename(currentFile));
+        if (!fs.existsSync(destFile)) {
+          fs.renameSync(currentFile, destFile);
+          currentFile = destFile;
         }
+        // Remove old album dir if now empty, then old artist dir if also empty
+        const oldAlbumLeft = fs.readdirSync(oldAlbumDir).filter(f => !f.startsWith('.'));
+        if (oldAlbumLeft.length === 0) {
+          try { fs.rmdirSync(oldAlbumDir); } catch {}
+          try {
+            const oldArtistLeft = fs.readdirSync(oldArtistDir).filter(f => !f.startsWith('.'));
+            if (oldArtistLeft.length === 0) fs.rmdirSync(oldArtistDir);
+          } catch {}
+        }
+      } catch (e) { console.error('[edit-metadata] move:', e.message); }
+    } else if (album && path.basename(oldAlbumDir) !== sanitize(album)) {
+      // Only album name changed (no artist move) — rename album folder in place
+      const renamedDir = path.join(oldArtistDir, sanitize(album));
+      if (!fs.existsSync(renamedDir)) {
+        try { fs.renameSync(oldAlbumDir, renamedDir); }
+        catch (e) { console.error('[edit-metadata] folder rename:', e.message); }
       }
     }